The present invention relates to an image forming apparatus and a laser drive control method in the apparatus for image formation by scanning laser light, modulated in accordance with an image signal, on a photosensitive body.
As a conventionally known image forming apparatus to print-output an image data, a laser-beam printer drives a laser to emit light by a drive signal modulated based on image information, scans the laser light on a photosensitive body and thereby forms an electrostatic latent image on the exposed photosensitive body, and then records a desired image through a duplication process including development, transfer and fixing. When the laser is driven to emit laser light, auto power control (APC) is performed to reduce changes in light emission output due to changes based on temperature characteristic and secular change of the laser.
This control is made by detecting a part of the laser light emitted from the laser by using a photosensor, converting a current value outputted from the photosensor in correspondence with the detected light amount into a voltage, and outputting the voltage as a voltage PDOUT via a buffer and an amplifier. The light emitting characteristic of the laser is obtained from the voltage PDOUT, which is obtained to detect a part of the laser light emitted from the laser drive by a PWM pulse.
FIG. 9 is a line graph showing the relation between the laser light amount with respect to a PWM pulsewidth corresponding to the drive signal to drive the laser and the voltage PDOUT to monitor the laser light amount.
At a point A in FIG. 9, the laser starts light emission in correspondence with a PWM pulse having a specific width. When the light emission is started, the amount of emitted light linearly increases with respect to the pulsewidth. Then at a point B, the pulsewidth has a value greater than a predetermined value, and the light amount increases with a greater gradient with respect to the pulsewidth. Thereafter, the light amount is saturated to a value Psat. This means the light amount becomes abruptly beyond Pmax, even if the laser drive current is turned off, as injection carrier density becomes off with a time constant, it is influenced by the pulse for the previous pixel. Generally, the PWM pulsewidth is adjusted such that the pulsewidth is a minimum pulsewidth when the image data has a minimum value at the point A, and the pulsewidth is a maximum pulsewidth when the image data has a maximum value at the point B.
The points A and B in FIG. 9 differ according to minute difference in characteristics of respective lasers, laser drive current circuits to drive these lasers, environmental temperatures around the semiconductor lasers, and the like. Accordingly, the pulsewidth must be controlled at every occurrence of change in the environment of laser unit as a combination of laser and laser drive current circuit. Generally, the voltage PDOUT to monitor the laser light amount is measured at predetermined timings and the maximum and minimum pulsewidths are controlled.
The above control method is based on the assumption that the amount of emitted laser light and the voltage PDOUT are proportional. However, as the voltage PDOUT is obtained by outputting a voltage, based on the output of a photodiode which has detected the laser light amount, through circuits such as a buffer and OP amplifier, the influence by input offset voltage components existing in the buffer and the OP amplifier cannot be ignored.
Originally, the characteristic of light amount with respect to PWM pulsewidth and the PDOUT characteristic with respect to PWM pulsewidth should be in a similitude relation, however, as shown in FIG. 9, at the point A in the light amount characteristic for minimum pulsewidth control, the PDOUT characteristic with respect to the PWM pulsewidth is shifted to a point Axe2x80x2 due to the input offset voltage components. Similarly, at the point B for maximum pulsewidth control, the PDOUT characteristic with respect to the PWM pulsewidth is shifted to a point Bxe2x80x2. Accordingly, when the pulsewidth control is performed to obtain the PWM minimum and maximum pulsewidths while monitoring the voltage PDOUT, the light amount characteristic of the laser light does not correspond with the pulsewidth.
The present invention has been made in consideration of the above conventional art, and has its object to provide an image forming apparatus and a laser drive control method in the apparatus for driving a laser in accordance with the amount of laser light emission without influence of the above-described offset components.
Further, another object of the present invention is to provide an image forming apparatus and a laser drive control method in the apparatus capable of accurately detecting the amount of light emitted from the laser and performing drive pulsewidth control in correspondence with the detected light amount.
In order to attain the above described objects, an image forming apparatus of the present invention comprising the structures as follows.
An image forming apparatus for forming an image by modulating laser light in accordance with an image signal, comprising: detection means for detecting laser light emitted from a laser diode and generating a detection current corresponding to the intensity of the laser light; detection voltage generation means for generating a detection voltage based on the detection current outputted from the detection means; and current supply means for adding a predetermined current to the detection current generated by the detection means and applying the detection current to the detection voltage generation means so as to compensate for the detection voltage generated by the detection voltage generation means.
In order to attain the above described objects, a laser drive control method of the present invention comprising the steps as follows.
A laser drive control method in an image forming apparatus for forming an image by modulating laser light in accordance with an image signal, comprising: a detection step of detecting laser light emitted from a laser diode and generating a detection current corresponding to the intensity of the laser light; a detection voltage generation step of generating a detection voltage based on the detection current outputted at the detection step; and a current supply step of adding a predetermined current to the detection current generated at the detection step so as to compensate for the detection voltage generated at the detection voltage generation step.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same name or similar parts throughout the figures thereof.